Lancets are used in the sense of the invention as blood withdrawal systems which are used to puncture an appropriate part of the body in order to remove a small amount of blood from a part of the body for analytical or diagnostic purposes. The blood is frequently drawn from a finger or earlobe. In medical practices lancets are used for this which are inserted into an appropriate part of the body by a doctor or laboratory staff either manually or with the aid of a simple apparatus. The lancet must of course be sharp and sterile. Otherwise there are no especially stringent requirements for medical practices since blood is collected from individual patients at large time intervals and the puncture is carried out by specially trained staff. Nevertheless the puncture is frequently associated with considerable pain.
If the blood withdrawal system should be suitable for operation by the patients themselves, this makes much higher demands on the system especially with regard to pain-free and reliable blood withdrawal. The independent use of blood withdrawal systems by patients is carried out especially for so-called home monitoring. In this case the aim is to enable groups of patients that are at special risk to regularly monitor certain analytical values of their blood. This applies among others to diabetics who frequently and regularly check their blood sugar level and adapt it to the requirements by injecting insulin. The insulin requirement depends among other factors on the food intake and physical activity and must be kept as continuously as possible within certain set limits. This is important for the health of the patients and to avoid serious secondary damage such as loss of sight and amputation of parts of the body.
Blood withdrawal systems have been used for a long time which consist of a lancing device and associated lancets that are specially adapted for the respective instrument. A housing of the lancing device contains a lancet drive which mechanically punctures the skin with a lancet. A spring is usually used as a drive element for the lancing movement. In the initial phase of development very simple constructions were commonly used in which the lancet was directly attached to one end of a pressure spring arranged in an elongate housing. Such a blood withdrawal device is disclosed for example in U.S. Pat. No. 4,469,110. However, the use of the described systems show that in particular the requirement for a less painful blood withdrawal cannot be fulfilled.
Recently small, simple-to-operate and relatively low-cost analytical systems have been developed to reduce the pain of puncture which are usually composed of blood test strips and an accompanying evaluation instrument. Such modern blood withdrawal systems are intended to ensure that blood withdrawal is as painless as possible for the patient and is simple to handle. Therefore in the recent prior art numerous different blood withdrawal systems are known which are suitable for generating the incisions required for blood collection in a simple and relatively painless manner.
Such blood withdrawal systems usually comprise a housing with an exit opening for the tip of a lancet and a lancet holder for holding the lancet which can be moved relative to the housing along a predetermined straight puncturing path. The lancet holder is moved during the puncturing and return movement by a lancet drive which has an elastic drive element which is usually a metal spring. In a first position in which the elastic drive element is in a tensioned state, the lancet holder is usually locked by means of a locking device. After such a locking device is released, the elastic drive element relaxes so that the movement of the drive element is converted into a puncturing movement of the lancet drive which moves the lancet held by the lancet holder at high speed along the predetermined lancing path in a puncturing direction until the tip of the lancet emerges from the exit opening of the blood withdrawal system. A wound is generated in a part of the body pressed against the exit opening. Immediately afterwards the lancet is usually retracted by the lancet drive into the housing. The drive units of the modern lancing devices which reduce pain due to a high lancing speed and the retraction of the lancet after the lancing consequently ensure that in the case of the modern lancing devices the blood withdrawal is considerably more pleasant for the patient.
Such blood withdrawal systems of the prior art are described for example in U.S. Pat. Nos. 4,442,836, 4,535,769 and 4,924,897. In the design described in U.S. Pat. No. 4,924,879 a spiral drive spring acts on a wheel whose rotation is converted by means of a lever connected to the wheel into a lancing and retraction movement. It is claimed that the pain is reduced by among others the fact that this movement proceeds very rapidly. However, the construction using precisely manufactured metal parts is complicated and involves many parts. Another disadvantage is that the lancet emerges from the exit opening when the lancet drive is tensioned resulting in a risk of injury for the user.
European Patent No. 0 582 226 describes another lancing device which is suitable for withdrawing blood samples. A piston driven by a spring means is movably mounted within the lancing device. A lancet which emerges through an opening provided in the housing during a lancing process is located at one end of the piston. The outer periphery of the piston has wings of only limited strength which bear against the housing wall of the lancing device. When the lancing process is carried out the piston is moved in the puncturing direction by means of the spring elements thereby destroying the wings of the plunger so that they no longer rest against the housing.
A disadvantage of this prior art device is that the spring elements for driving the piston have to apply sufficient force to first destroy the wings of the piston before the piston can be moved in the lancing direction. Furthermore if the wings are not completely destroyed, this can cause frictional effects when the piston moves within the housing in the lancing direction. Hence the conditions of a lancing process are changed depending on the frictional effects so that undefined force curves act during the lancing process. This influences among others the puncturing speed of the lancet into a part of the skin so that the patient has to expect different levels of pain caused by the puncturing.
In addition the lancet has a sterile protector which has to be removed from the needle tip before use. However, the wings may be damaged as a result of a strong pulling motion and this may accidentally trigger a lancing process. Since the described mechanism is only suitable for disposable lancing devices and it is not possible to retension the lancet due to the destroyed wings on the piston, the unused lancet has to be consequently discarded.
Furthermore U.S. Pat. No. 4,416,279 discloses a blood withdrawal system in which the lancet holder is moved along an inclined ramp located in the housing in an opposite direction to the lancing direction by rotating an external part of the housing. This movement tensions a spring member which serves as the drive element for the lancet holder. The sloping surface protruding into the housing has a plateau on which the lancet holder can be temporarily held after a first rotary movement of the external housing member. In this position the lancet holder is pulled back in the opposite direction to lancing to such an extent that the lancet can be removed from the lancet holder and exchanged. By continuing the rotation the lancet holder is moved further along the sloping surface in the opposite direction to lancing, further tensioning the spring element until the lancet holder is moved beyond the sloping surface and is driven by the force of the spring in the puncturing direction. The lancet tip emerges from the housing.
A disadvantage of the prior art is that the rotational movement of the outer housing member makes it more difficult for the user to handle the blood withdrawal system. In order to tension the blood withdrawal system and simultaneously position the finger pad at the exit opening of the blood withdrawal system, the patient has to position his finger in a holder provided. The holder also serves to fix the position of the blood withdrawal system on the finger tip while the user rotates the outer housing member with the other hand. The said blood withdrawal system proves to be extremely unwieldy and is very difficult to operate particularly by elderly persons.